Spectrum 2+A blinking power issue

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ziggaboogi
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Spectrum 2+A blinking power issue

Post by ziggaboogi »

Hello!

I'm hoping to get some advice on what to check next on a spectrum 2+A that I got untested online.

As of now the issue is that when I plug in the PSU the LED will flash on for a second and the screen will show a thick purple bar in the center and then the unit turns itself off and repeats those symptoms.

For now I have
-checked the voltages on the PSU pins and they were fine
-reseated every chip that was socketed on the board
-am currently making my way through the schematic to see if there are any parts I should check first

At this point I'm obviously not trying to diagnose it further until I can figure out what's causing this power issue. Any help would be greatly appreciated!
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1024MAK
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Re: Spectrum 2+A blinking power issue

Post by 1024MAK »

If you mean the red power LED on the computer is blinking, then there is a power issue.

Either there is a fault with the PSU and it is no longer capable of supplying the power required by the computer.
Or there is a fault in the computer causing it to draw more power than the PSU can cope with.

You need to take the top off the computer, then measure the actual voltage on the +5V supply.
Next, disconnect the internal cassette deck, then measure the actual voltage on the +5V supply again.

Report your results back here.

Mark
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

Okay will this is embarrassing.

It turns out that I wired up the PSU incorrectly when following this guide....

https://spectrumforeveryone.com/technic ... -machines/

For the pinout I had the wires mirrored as I didn't see the part that said the diagram was from the solder side of the DIP-6...

Which means that I definitely applied the wrong voltages to certain components.... After fixing the PSU pinout to be correct, I tried firing it up this time and it is slightly different but still having the similar symptoms.

Now the unit shows a green bar in the middle of the screen for a flash and then resets. When checking the voltages out on the main board I'm definitely not seeing the correct voltage for the +5 as it is fluctuating around 1v and it bounces up and down quite a bit.

While the PSU is still suspect, I'm assuming that I had to have blown a component or two when I had it wrong as that means I was applying 12v to the 5v line.....
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Re: Spectrum 2+A blinking power issue

Post by Guesser »

I'm amazed the gate array does anything at all if you gave it 12 volts :shock:
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

Yeah I messed up hard on this one.

My best hope is that something was already faulty to begin with even before the PSU mess up that would save any of the hard to replace components.

Or that something fairly easy to replace was burned up and no proprietary parts were damaged.

Man I wish finding ZX stuff in the US was a lot easier : /

Edit:
I did actually order a new PSU for a ZX Spectrum +2A from this website as I don't want to risk any further damage with a DIY solution http://www.keelog.com/zx-spectrum-psu-m ... y-us-plug/

Depending on how it behaves with this one will probably tell me if there's something wrong with my DIY PSU or if something is in fact broken in the zx.
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1024MAK
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Re: Spectrum 2+A blinking power issue

Post by 1024MAK »

If the +5V supply is less than 4.75V then we still have the same problem. Is the PSU not capable of supplying enough current (power). Or is there something breaking down on the computers PCB that’s shorting out.

What type of PSU are you using and what are it’s specifications?

If the +12V output is relatively low power, it may have current limited and not much damage to the computers circuitry may have occurred. However if not, then various chips are likely to have self destructed :(

If the gate array (that square one with lots of small pins) has died, the machine is a write off, as there are no known spares. The other chips are either still in production or are available on trading sites (although the sound chip is difficult to get as there are so many faulty or fakes around).

In order to fault find this, what you now need are:
  1. A good quality regulated bench power supply with adjustable current limit. Capable of 5V, with an output current of 2A or more (these normally have adjustable voltage outputs as standard),
  2. A good quality +12V regulated power supply rated at 250mA or more,
  3. A good quality multimeter that has 2A and 10A DC current ranges,
  4. A second multimeter so you can monitor the voltage at the same time that your first meter is monitoring current,
  5. A logic analyser, at least 16 channels, but 24 channels would be better,
  6. Soldering and desoldering tools,
  7. Lots of spare time.
A good quality oscilloscope (analogue bandwidth of 50MHz or better) may also be needed.

If you have access to a thermal imaging camera, that may help to speed up the process.

The first order of business is to supply just the +5V supply rail with +5V via item 1 in the above list, the output of which is routed via a multimeter (item 3) switched to the 10A range. Adjust the PSU voltage to exactly +5V and adjust the current limit to minimum. Connect up to the computer. Attach item 4 to monitor the voltage that is going to the computer. Slowly increase the current limit towards 1A. Has the voltage gone up? Continue to increase the current to 1.5A. Now check all the semiconductors (chips, transistors and diodes) to see if any are warm or hot. If there are any that are very hot, then note which ones, and turn the power off. Cut them out of the PCB. Now repeat the above procedure again.

If nothing is getting hot, keep increasing the current limit, but stop if you get to 2A. Keep checking the temperatures.

Note that it’s normal for the Z80 to run slightly warm and some other chips to also run slightly warm.

Mark
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Re: Spectrum 2+A blinking power issue

Post by dfzx »

I enjoy following these hardware problem hunts. Well, trying to follow them anyway. :)

I was a bit confused by this bit though:
1024MAK wrote: Sun Aug 23, 2020 10:14 am Attach item 4 to monitor the voltage that is going to the computer. Slowly increase the current limit towards 1A. Has the voltage gone up?
You've got the +5V coming from a bench power supply, so that should be stable, right? So as you increase the current the computer is allowed to draw, what voltage are you asking has gone up? And what if it has, or hasn't?
Derek Fountain, author of the ZX Spectrum C Programmer's Getting Started Guide and various open source games, hardware and other projects, including an IF1 and ZX Microdrive emulator.
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Re: Spectrum 2+A blinking power issue

Post by Guesser »

dfzx wrote: Sun Aug 23, 2020 12:41 pm You've got the +5V coming from a bench power supply, so that should be stable, right? So as you increase the current the computer is allowed to draw, what voltage are you asking has gone up? And what if it has, or hasn't?
With the current turned right down it would go into limit as soon as you attach the computer so the voltage would fall.
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Re: Spectrum 2+A blinking power issue

Post by 1024MAK »

With a regulated bench power supply that has current limiting, it will either regulate the current or regulate the voltage (and will switch automatically between these modes as required).

If the load (in this case the suspect +2A board {or +2B board}) requires more current than it is getting, then the PSU will be in current limiting mode. As the current is being limited, the voltage can’t be at the set output voltage of the PSU.

If you have a hose pipe fed from your mains water tap, normally the pressure is good enough so that at the end of the hose, the water squirts out a good distance. But if someone puts their foot on the hose (thus limiting the amount of water that can flow), the water no longer squirts out the same distance, despite the pressure in the tap being the same.

Or we can use some maths and Ohms law. Say a load normally draws 1.5A of current at 5V DC.
Using Ohms law, we can work out that the equivalent resistance of the load is 3.33 ohms.

V
— = R
I

Where
V is the voltage, I is the current (in Amps) and R is the resistance in Ohms.

5
— = 3.33
1.5

So if we only supply 1A, after rearranging the equation, we get this:

I x R = V

1 x 3.33 = 3.33V

Unfortunately semiconductors are far more complex than a ‘simple resistive load”, so the actual voltage may be different (some devices may not turn on for example).

The why, well it may well be that one or more semiconductor devices have self destructed, often this results in them going partly short circuit. But at the same time, we don’t want to kill anything else.

If the voltage does not go up, then we definitely have a short circuit. If the voltage goes up near to +5V, then there may not be any damage at all.

Mark
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

The PSU I was using is the same model as the one in the guide so it would have the following specs.

+5v: 3A
+12v: 2A
-12v: 0.2A

Thank you for the run down of the equipment I would need. While I do have some of it like a the soldering/desoldering equipment and multimeters, I really do need to look into getting a benchtop power supply and an oscilloscope (also learning how to use one). But those were already on my "get for my workbench" list.

For now my best hope is that something was wrong with the PSU I used as I wasn't able to verify the actual max currents on this thing, only verifying the voltages.

Anyways I'm hoping that the new PSU should be more reliable and again fingers crossed that I didn't break the gate array!
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Re: Spectrum 2+A blinking power issue

Post by dfzx »

1024MAK wrote: Sun Aug 23, 2020 4:39 pm With a regulated bench power supply that has current limiting...
Thanks for the excellent and comprehensive explanation. :D
Derek Fountain, author of the ZX Spectrum C Programmer's Getting Started Guide and various open source games, hardware and other projects, including an IF1 and ZX Microdrive emulator.
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

So the new power supply came and after verifying voltages I did try powering on the unit again.

Still has the blinking red light issue which tells me I probably did fry something on the inside. This power supply also has an LED installed which also blinks when the unit blinks. I'm leaning towards that I may have fried a chip and something is bridging that isn't supposed to which is making the PSUs reset.

I'll be doing some more testing but I did notice a burning circuit type smell near IC 12 and 13. Would it be worth ordering replacements for those two while I dig further around the board?
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Re: Spectrum 2+A blinking power issue

Post by 1024MAK »

So it’s likely that the PSU is shutting down when the output current exceeds its ‘trip’ level. Then it automatically restarts.

If any of the chips, transistors or diodes are getting too hot to touch, then they are damaged. They should be removed.

Either desolder them (if you have the correct desoldering equipment and are good at removing chips from double sided plated through boards), otherwise use electronic snips to cut the pins of the suspect chip as near the plastic body as you can. Then you can remove one pin at a time, which is much easier and reduces the likelihood of damage to the PCB tracks and the plated through holes.

Don’t replace any chips until the supply voltage is a nice stable 4.75V to 5.25V.

Mark
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

Thanks for the advice Mark.

So I ended up removing both IC 12 and 13 with my desoldering gun as the smell still made me suspicious and for now I've just put in sockets as i wait for the replacement chips to arrive.

The good news is that now the power supply is nice and stable and giving just a little over 5V!

Also now the system power LED is no longer blinking nor is the PSU LED. Here's hoping that those chips were "sacrificial" in a sense and that them causing the PSU to keep tripping saved all the other components from frying.

Anyways once I get the replacements for IC 12 and 13 I'll try hooking it up and doing a full test on it and come back with an update.
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Re: Spectrum 2+A blinking power issue

Post by Ast A. Moore »

You can certainly run your Speccy without IC12 or IC13. Those are just RS-232 driver/receiver and are only needed if you want to use the AUX/MIDI sockets.
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

So I did try connecting it to a screen and have found a few things.

The screen is in fact getting the RGB video but it's just a black screen for now.

The four DRAM chips of IC 3 ~ 6 are getting very hot.

After checking out the transistors, it looks like TR201 has a short as it's NPN but it's seeing a voltage drop of zero on one side.

I've ordered replacements for TR201 and will keep looking around the board for other possible faults.
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Re: Spectrum 2+A blinking power issue

Post by ziggaboogi »

SUCCESS!

So after changing the four DRAM chips it booted right up!

I was even able to get the tape loader to work on a copy of Wolfan!

So in total due to my wiring mistake I had broke the following

IC12, IC 13, 4 x DRAM and a transistor.

The lesson learned here is to not just triple check your wiring work, but to also triple check the diagram itself!

Thanks again to all the advice on here! I am extremely fortunate that nothing hard to replace had been destroyed due to my wiring mistake.

Image

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1024MAK
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Re: Spectrum 2+A blinking power issue

Post by 1024MAK »

Well done on getting it working. I’m sure you are both relieved and delighted that you did not give up!

Have fun now :D

Mark
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